Clinical pharmacokinetics of H1-receptor antagonists (the antihistamines).

This article reviews clinical pharmacokinetic data on the H1-receptor antagonists, commonly referred to as the antihistamines. Despite their widespread use over an extended period, relatively little pharmacokinetic data are available for many of these drugs. A number of H1-receptor antagonists have been assayed mainly using radioimmunoassay methods. These have also generally measured metabolites to greater or lesser extents. Thus, the interpretation of such data is complex. After oral administration of H1-receptor antagonists as syrup or tablet formulations, peak plasma concentrations are usually observed after 2 to 3 hours. Bioavailability has not been extensively studied, but is about 0.34 for chlorpheniramine, 0.40 to 0.60 for diphenhydramine, and about 0.25 for promethazine. Most of these drugs are metabolised in the liver, this being very extensive in some instances (e.g. cyproheptadine and terfenadine). Total body clearance in adults is generally in the range of 5 to 12 ml/min/kg (for astemizole, brompheniramine, chlorpheniramine, diphenhydramine, hydroxyzine, promethazine and triprolidine), while their elimination half-lives range from about 3 hours to about 18 days [cinnarizine about 3 hours; diphenhydramine about 4 hours; promethazine 10 to 14 hours; chlorpheniramine 14 to 25 hours; hydroxyzine about 20 hours; brompheniramine about 25 hours; astemizole and its active metabolites about 7 to 20 days (after long term administration); flunarizine about 18 to 20 days]. They also have relatively large apparent volumes of distribution in excess of 4 L/kg. In children, the elimination half-lives of chlorpheniramine and hydroxyzine are shorter than in adults. In patients with alcohol-related liver disease, the elimination half-life of diphenhydramine was increased from 9 to 15 hours, while in patients with chronic renal disease that of chlorpheniramine was very greatly prolonged. Little, if any, published information is available on the pharmacokinetics of these drugs in neonates, pregnancy or during lactation. The relatively long half-lives of a number of the older H1-receptor antagonists such as brompheniramine, chlorpheniramine and hydroxyzine suggest that they can be administered to adults once daily.